This application relates to maintaining automatically biasing an amplifier, and more specifically to automatically tuning the current of the amplifier so that the amplifier will operate at an optimized mode.
Amplifiers are designed using several operating classes including those known as Class A, Class B, Class AB, and Class G. Each of these classes addresses one or more particular aspects of desired amplifier behavior but all of them have constraints or tradeoffs that are substantially unavoidable.
Ethernet physical layer normally employs class AB amplifier at the output stage. However, class AB might not be the best deployment as it normally has higher distortion as compared to class A. Although class AB tends to have better efficiency.
Due to the wire bonding, packaging and cable length, the output pins of the amplifier might see some stray inductance. The value of the stray inductance would be higher if the amplifier's bonded wire is long, if the amplifier packaging is large or if the amplifier is driving a long cable.
When the inductance is high, a pure class AB amplifier might distort signals with a large overshoot or undershoot. This impairment can cause signal integrity issue. The result is more packets loss over the signal line, especially when the cable is long. When the stray inductance is high, operating the amplifier at or near class A could improve the amplifiers signal integrity as class A has much better signal quality at the expense of poorer efficiency. Operating at class A reduces high overshoot/undershoot when the inductance is high. However, class A amplifiers can easily be overloaded by unusual and excessive output loads. Operating the amplifier at class B can result in a generated crossover distortion as the amplifiers output signal changes its polarity.